Development of energy sources that are alternatives to fossil fuels has become more and more important over the years. One such alternate source is solar energy wherein photovoltaic cells or the like directly convert solar energy into useful electrical energy. Typically, a plurality of these photovoltaic cells are encased between a transparent sheet (e.g. glass, plastic, etc.) and a sheet of backing material to form a flat, rectangular-shaped module (sometimes also called “laminate” or “panel”) which, in turn, is installed onto the roof of an existing structure (e.g. a house, building, or the like) to provide all or at least a portion of the electrical energy used by that structure.
A majority of photovoltaic cells are comprised of silicon-based, crystalline substrates. These substrates can be wafers cut from ingots of silicon or from ribbons of silicon where the ingots or ribbons are “grown” from batches of molten silicon. In the early development of ribbon crystal growth and solar energy, several apparatuses were developed to produce silicon bodies or substrates with good crystalline quality. Unfortunately, however, most of these ribbon growth apparatuses focused too intently (a) on producing perfect crystalline quality of the ribbon (e.g. the net shape of the substrates which were typically between 200 and 400 microns in thickness) or (b) on perfecting the combination of melt and growth necessary for a continuous growth operation. This resulted in only small crystalline areas being grown or, alternately, the entire operation was unstable due to dual melt isotherms in contact with the growth region.
As part of these efforts, significant focus was also placed on the application of cooling gas, primarily helium, to the growth region to effect the proper heat extraction required for the desired growth of the crystals. However, difficulties arose in maintaining the proper volume of cooling gas during a growth operation. That is, insufficient gas flow did not allow enough heat to be extracted during the growth of the crystalline substrate to enable sufficiently high growth rates to be readily economically competitive. On the other hand, merely increasing the flow of cooling gas to a rate sufficient to achieve the required extraction of heat resulted in the increased gas flow disrupting the melt surface of the substrate thereby preventing the formation of a desired flat sheet of crystal. Still another attempt to correct this problem involved decreasing the heater input power but, unfortunately, this resulted in poor thermal gradient control; i.e. the ability to maintain a stable crystal growth front without a proclivity to form dendrites.
More recently, other approaches have been proposed to overcome some of the above-described problems related to the growth of a silicon-based, crystalline substrate. For example, U.S. Pat. No. 4,329,195 describes a technique for growing a thin and wide substrate at relatively high rates by using a cooling gas (i.e. mixture of argon and hydrogen or helium) and a seed crystal to start the growth of the substrate. However, the supply of molten silicon is replenished by a silicon feed rod which, in turn, is positioned into same melt zone that produces the nearby crystalline sheet, thereby creating an undesirable dual melt isotherm dilemma which presents control problems. Another technique involves directly contacting the substrate with a heat sink to control the extraction of heat from the substrate during growth; see U.S. Pat. No. 3,681,033 and “Float Zone Silicon Sheet Growth”, C. E. Bleil, Final Report-DOE Grant No. DE-FG45-93R551901, Sep. 23, 1993 to Dec. 31, 1996 which may cause unevenness in the surface of the substrate.
Further, many prior techniques for forming crystalline substrates rely on the formation of a meniscus to act as the sole support of the growth process; see U.S. Pat. No. 2,927,008. However, many of such free-standing meniscus shaping and related techniques use graphite or silicon carbide dies for ribbon definition and accordingly, have the difficulty of providing a stable and uniform ribbon thickness under varying conditions. Also, continuous and direct contact with one of these carbon sources contributes to melt contamination and limits the performance of an eventual solar cell. These types of techniques also rely on vertical pulling from the melt and since heat will be lost along the length of the growing ribbon perpendicular to the growth front, the total amount of heat which can be extracted is severely limited.